A light-emitting diode backlight unit (LED BLU) which actually emits light is one of the most important elements of a liquid crystal display (LCD) TV.
According to one of methods for manufacturing a white LED BLU, red (R), green (G) and blue (B) LED chips with a small value of full width at half maximum (FWHM, a difference between a pair of wavelength values on a relative spectral distribution graph at which the dependent variable is equal to half of a maximum value thereof, with a unit of nm) are combined with each other.
However, according to this method in which red, green and blue LED chips are combined to manufacture a while LED BLU, the number of LED chips used is large and an additional feedback system is required to obtain a white color, causing an increase in the manufacturing cost.
A white color may also be obtained by combining a blue LED chip and a yellow (Y) phosphor that emits light having a wavelength with a large FWHM. According to this method, the number of LED chips is reduced to one third of the number of LED chips required for the above-mentioned method, and the feedback system is not necessary, so that the manufacturing cost of a BLU may be reduced. However, since a phosphor with a large FWHM is used, a color filter (CF) is necessarily required to provide red, green or blue color with high color purity for a corresponding spectrum. Since the CF blocks light of a wide wavelength range, light extraction efficiency of a device and the color purity are low, limiting color reproducibility.
Therefore, researches have been actively conducted to improve the color reproducibility and resolve the issue of low light extraction efficiency due to blocking of light of a wide wavelength range by a CF by replacing a typical phosphor having a large FWHM with a quantum dot having a small FWHM.
A quantum dot is a particle in which a nano-size II-IV semiconductor particle forms a core. Such a quantum dot emits light when an electronic exited to a conduction band falls back to a valence band.
Regarding the development of lighting based on a quantum dot phosphor, the quantum dot may emit light of various wavelengths according to a synthetic material thereof and a quantum dot size. Furthermore, the FWHM of a beam of emitted light is adjustable. Moreover, due to physical characteristics of the quantum dot, such as a small FWHM and a beam of emitted light of various wavelengths, a lighting apparatus that emits light similar to the light of the sun can be developed, and a BLU with high color reproducibility can also be developed.
However, when quantum dot phosphors are applied to such various fields, various expensive optical functional films are required to obtain uniform flat light within a short distance as the dot phosphors are applied to thin-film-type structures.